Order wire circuit for carrier transmission systems



Feb. 26, 1946.

ORDER WIRE CIRCUIT FOR CARRIER TRANSMISSION SYSTEMS P. G. EDWARDS Filed June 27, 1942 4 Sheets-Sheet 1 AMP.

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m vavron R6. EDWA R05 ATTORNEY Feb. 26, 1946. G, EDWA DS 2,395,540

ORDER WIRE CIRCUIT FOR CARRIER TRANSMISSION SYSTEMS Filed June 27, 1942 4 Sheets-Sheet 2 /o p5 F/G.4 25

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INVENTOR RGEDMMRDS By g /2 AFT'ORNEK Feb. 26, 1946. R EDWARDS] 2,395,540

ORDER WIRE CIRCUIT FOR CARRIER TRANSMISSION SYSTEMS Filed June 27, 1942 4 Sheets-Sheet 3 F/G.9 ""Q as WEST 60 6 w w 4%. m

lNVE/VTOR P. G. EDWARDS ATTORNEY Feb. 26, 1946. P. G. EDW RDS 2,395,540

ORDER WIRE CIRCUIT FQR CARRIER TRANSMISSION SYSTEMS Filed June 27, 1942' 4 Sheet-Sheet 4 ,4 1 Am ll 5 0 2AM). C

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A T TORNEV Patented Feb. 26, 1946 IT'EPD sures RORF'LCE .ORDER :WIREaGIRCUIT FOR CARRIER TRANSMISSION. SYSTEMS EPaul GJEdwardg Verona, N. 21., assignor to Bell Telephone Laboratories,

Incorporated, i'New York,-"N. Ye,=ia' corporation mt Newtyork ..'Applieation 31111627,

S ClaimS.

.This inventionz'relates-tonatsignal transmission system, andmore particularly to an-rorder wire circuit connectedracross the input and output terminals of certain amplifiersoi each one-way.pair of "a tour-wire circuit embodied in suchesystem.

-In :the operation of.a.-long telephone circuit extending through -:a ,plurality .of repeater stations, .circuits for. communicatinglbetween.certain repeater .-stations are required toTacilitatemaintenance of: the telephone circuits. .Such maintenancecircuitsareeknown as order wires. -Originally, long voice circuits and .carriertsysterne employed ;telegraph channels for the .order .wire

service. When. longvoice frequency cable circuits .were introduced. into zcommercialnuse, voice order wire circuits were: obtained-.by-utilizing monitorling-windings of. line repeaters,.togethepwith' idle lines. -.In multiconductorcablersystems, individual conductor pairs were reserved for .order wire .service.

With .the Eintroduction .of a .cable carrier ..system inwhich a plurality. of channels were pro- .vided on individual.conductor pairs .ofa multiconductor .cable, the telephone circuits were extended. to longerdistances and more repeaterjsta tions were. reqturedlforas-giveni' length .er these circuits. .This.. meant lthat. an increased number .of telephone. subsets would probably be" bridged wire orderwire circuit was "selected" to -minimize echoes,".andwas'providedby utilizing'an individual.conductorpairineachof twooppositely transmitting cables.

' In the earlier time offour-wire, order wire circuit, a'tt'elephone subsetv/as"bridged across the output terminals of corresponding one-wayamplifiers embodied in a; four wire "'circuiti. through a transformer. applied toleachtone-way conductor pair. This constituted a single bridgetelephone tor'der wire. With suchztelephoneisubsetPrepared 1942,"Serial"No. 448,740

(ClwI'Z'Q-l-lZO) for talking, a relatively low transmission lossand echo .pathextended through the -two transformerseand .the telephone subset, Lbeing of the order of '18 decibels in one example. Due tothe mul- -5 tiplicity-of telephone .subsets' bridging the "fourwire order icircuitcon ailong "telephone circuit, echoes-would.be.relatively'large. 'Another trans- .mission..loss would be encountered inthe brid ing telephone.subsets. .per. se. A further (disad- .10 vantageof the .singlebridge order'wirecircuits resided in the lowlevelof outgoing speechcurrent on the transmission i line. As "the single bridge teleiihonesubset is applied; across the ;out- .put terminals of corresponding one-'Wayampli- 11-5 lfiers; the outgoing speech current .from' the telephone subset was .found to be approximatelyw decibels be1ow the normal level of Signals at'suc'n output terminals. As the order'wire circuitswere embodied in the same'cable' with the through cir- .-20.cuits carrying other .v-oice currents, crossetal'k .fromthe'ilatter. circuitsito the order wirecircuits was Ifound to. beexcessive.

"The present invention therefore contemplates an order wire circuittending"to"1minimize"echo .25 andv singing effects and...otheritransmission losses in cable carrier systems extending over a length of the orderof one thousand mileswith main repeaterstations" located approximately at 'fiftymile intervals.

The main object .oftheimentionis topromote the e'fficiency of orderwireoperation in lon cable carrier telephone systems. Another object is to minimize'theinterfering efiectlof the operation of order wire circuits-on .35 associated repeaters.

A further object is to minimize "deleterious transmission effects. during both talkingand nontalking intervals ona'four-Wire order circuit.

In a specific embodiment in connection with a .40 pairiof oneeway'transmissionpaths each embodying 'at" least one one=way amplifier, the present I invent-ion con rprises a' bridge network embodying .-a telephone subset in Oneamm; a balanc-ing network man adjacent arm," and individual resistors 45 in eachzo'f thettwootherrarmsya hybrid-coil applicdtmeach of the two opposite diagonals of the 2 bridge networkjand circuit-means to-connect one Ihybrid coilto the output terminals of correspondingtone way amplifiers and the otherhy'brid coil 50 .to.the' input terminalsof the corresponding one- .Wayampiifiers. During'intervals when thetelephone'subset' and balancing network are not-being "used; suitable individual" resistors are-substituted tthereforin: the bridge network. .55 Other embodiments comprise a plurality of bridge networks applied simultaneously to both the input and output terminals of certain oneway amplifiers of the four-wire circuit; or a combination -of two hybrid coils and a bridge network; or a plurality of bridge networks efiectively connected across the spaced two-wire circuits; or a plurality of multiwinding repeat coils connected across eitherthe four-wire circuit or the spaced two-wire circuits.

The invention will be readily understood from Fig. 4 is a schematic circuit diagram delineate ing another embodiment of the invention including bridge networks only;

Figs. 5 and 8 are fragmentary circuit diagrams which may be embodied in both Figs. 4 and 6;

Fig. 6 is a schematic circuit diagram showing another embodiment of the invention including hybrid coils and a bridge network;

Fig. '7 is a fragmentary circuit diagram which may be substituted in Fig. 6;

Fig. 9 is a schematic circuit diagram showing another embodiment of the invention including bridge networks applied effectively to the spaced two-wire portions of a signal transmission line;

Fig. 10 is a schematic circuit diagram illustrating another embodiment of the invention including repeat coils only and applied across the fourwire circuit;

Fig. 11 is a schematic circuit diagram showing an alternate embodiment of Fig. 10 such that the repeat coils,are effectively applied between the spaced two-way circuits; and Figs. 12 and 13 are tables representing the currentflow obtainable in Figs. 9 and 10 under certain conditions. 7

Referring to Fig. 1, west-east path IIJ embodies one-Wayamplifier II, and east-west path I2 embodies one-way amplifier I3. These paths constitute one portion of the four-wire circuit of a cable carrier signal transmission system in which the individual east and west terminals, not shown, comprise two-Way, two-wire paths connected to the four-wire path through terminal hybrid coils in the usual manner. Such four-wire portion may be duplicated at main-repeater terminals, and at each auxiliary repeater terminals located between associated main repeater terminals.

A bridge network I6 embodying'individual resistance arms I1, I8, telephone subset I9 and baldiagonal I4 and 23 constitute the vertical 3 la whose terminal 32 is connected over leads 33,

33 and resistance pad 34 to the input terminal 35 of the W.-E. amplifier II, and whose terminal 36 is extended over leads 31, 31 and resistance pad and horizontal diagonals, respectively.

In the operation of Fig. 1, voice currents on the W.-E. and E.-W. paths Ill and I2 are taken from the output terminals 25 and 3| of the respective amplifiers II and I3 and applied through hybrid coil 2| to the telephone subset I9 inthe bridge network I6; and voice current originating in the telephone subset I9 in the bridge network I6 is applied through hybrid coil 3Ia to the input terminals 35 and 39 of the respective amplifiers II and I3 embodied in the W.-E. and lit-W. paths III and I2. As the gain of the amplifiers II and I3 is the same, the voice currents in the respective output terminals 25 and 3| of the individual W.-E. and E.W. paths I0 and I2 are always substantially at normal level,

Voice current at the input terminal 35 of the W.-E. amplifier II may reach the output terminal 3| of the E.-W. amplifier I3 over path I which further includes resistance pad 34.1eads 33, 33, hybrid coil 3Ia, leads 31, '31, resistance pad 38, input terminal 39 and. the E.-W. amplifier I3; or alternately over path II which further embodies W.-E. amplifier I I, output terminal 25, resistance pad 24, leads 23, 23, hybrid coil 2|, leads29, 29 and resistance pad 30. The net transmission loss in each of the paths I and II is, for the purpose of this illustration, approximately 55 decibels.

If the voice currents in the output terminal 3| of the E.-W. amplifier I3, clue to the paths I and 11 were in phase, the total of such voice currents would be twice the voice current in either path I or II or for the purpose of this illustration, of the order of 6 decibels greater than the 7 output of the E.-W. amplifier. I3. The net transmission loss of such voice currents in the output terminal 3| of the E.-W. amplifier I3 would be of the order of 49 decibels. As the leads of the output terminal 3| of the E.-W. amplifier I3 are reversed with respect to the leads of output terminal 25 of the W.-E. amplifier II, the voice currents due to the paths I and II combine at the terminal 3| in an out-of-phase relation. Hence, such voice currents substantially cancel each other. Due to some phase difierences of the paths I and II such voice currents may not cancel completely; but it was found that the interference efiect of such currents in the output terminal 3| was down approximately 65 decibels, in one example for the purpose of this illustration.

Voice current at the output terminal 25 of the W.-E. amplifier' II may reach the input terminal .39 of the E.-W. amplifier I3 over a path III embodying resistance pad 24, leads 23, 23, hybrid coil 2|, bridge network. I6, hybrid coil 3Ia, leads 31, 31, and resistance pad 38. Path III has atransmission loss for such current that the northe normal functioning of these amplifiers. For

the purpose of this illustration, such paths'comprise path IV including the terminals of the telephone subset I9 embodied in the bridge network I6, hybrid coil 3Ia, leads 33, 33 and resistance pad 34; and path V including output terminal 25 of the W.-E. amplifier II, resistance pad 24, leads23, 23, hybrid coil 2| and terminals of the telephone subset I9 embodied in the bridge network I6.

Thus, the complete transmission loss extending around the W.-E. amplifier II and comprising the paths "IV and V should be such magnitude as to permit substantially normal operation of the "W.-E. and Eh-W. amplifiers H and I 3. Likewise, the transmission loss obtaining with respect to E.-W. amplifier l3 through similar shunting and singing paths should be'such that normal operation of the latter is substantially provided for.

The arrangement of Fig. 1 further embodies anti-side'tone characteristics because of the turnover of the output terminal 3| of the E.-W. amplifier l3. Voicecurrent from the telephone subset-IQ is impressed on the hybrid coil 31a wherein this current divides such that one halfis transmitted over leads 33, 33 and resistance pad 34 to the input terminal 35 of the W.-E. amplifier l l, and the other half of such voice current is transmitted over leads 31, 3'1 andresistance pad 38 to the input terminal 39 of the E.-W. amplifierl3.

After these current portions pass through the respective amplifiers H and I3, thecurrent portions are then applied through respective resistance pads 24 and 30, leads 23, Hand 29, 29to the hybrid'coil 2! wherein their cancellation is efiected because the two current portions are now of opposite phase, due to the reversal of the output terminal 3! of the EAW. amplifier l3. The balance between the vertical terminal 1-4 and horizontal terminal 2--3 of the bridge network [6 in Figs. 1 and 2 isapproximately 0.9 decibel for the purpose of this illustration, and patching the telephone subset I9 and network 20 thereinto 'does not materially aifect such balance.

Fig. 2 shows a bridge network 40 which is similar to the bridge network [6 of Figs. 1 and 3 but the resistor 4| is substituted for the telephone subset 19 and the resistor 42for the balancing network 28 as above described, and may be substituted in Fig. 1 between the lines X- X and Y-Y during non-talking intervals on the order wire. The loss transmission characteristics specified above in connection with the paths I, II, III, IV, andV of Fig. 1 also substantially obtain in Figs. 1 and 2. This means that the normal operation of the W.-E. and EL-W. amplifiers II and E3 encounters substantially tolerable interference from the order wire circuit during both talking and non-talking intervals thereon.

Fig. 4 is a four-wire order circuit along the lines of the one illustrated in Fig. 1, and accomplishes substantially the transmission losses and echo and singing margins pointed out hereinbefore in connection with the latter figure. Referring to Fig. 4, the W .-E. and lit-W. transmission paths and 12, respectively, includingone-way amplifiers H and 13 are identical with the corresponding circuit elements of Fig. 1; vertical diagonal l-4 of resistor bridge network 45 is applied to the input terminal 35 of the W.-E. amplifier H while the horizontal diagonal 23 of the bridge network '25 is extended to the input terminal 39 0f the E.-W. amplifier [3; vertical diagonal 9-40 of the bridge network 46 is connected to theoutput terminal 3! of the E.-W. amplifier l3 while the horizontal diagonal ll--l2 of the bridge network 46 is impressed on the output terminal 25 of the W.-E amplifier H; telephone subset 4'! is applied across the vertical diagonal 5-6 of bridge network 48 while balancing network 49 is disposed across the horizontal diagonal '|--8 of the bridge network 48; arm 34 of the bridge network 45 is extended to arm 5-.1 of the bridge network 48 whilearm 6--8 of thelatter. bridge network'is extended to arm 9-4 I of the bridge network 46; and the-outputterminal 311ml? "the EAW. amplifier I3 is reversed relative to the output terminal of the W.-E. amplifier l l. Coupled coils 50 and 5| interposed in eachpath associated with theverticaland horizontal diagonals of the respective bridge-networks 45 and 46 serve to minimize noise effects by keeping the original voice circuit balanced with respect to ground. Resistance pads maybe interposed in Fig. 4 asin Fig. 1, if desired, to afford the desiredtransmitting and receiving gains. During non-talking intervals individualresistors may besubstituted for the respective telephone subset 41 and balancing network 49 by embodying Fig.5 in Fig.4 between the lines A-Aand B-B. It "may be shown substantially in the manner setforth'previously withregard to Fig. 1 that the normal operation of the W.-'E. and E.-W. amplifiers I l and I3 encountersuch transmission losses, and singing and echo efiects that tolerable interference from the order-Wire circuit of Figs. 4 and 5 with respect to the normal operation of Figs. 4 and 5 is substantially maintained during bothtalking and non-talking intervals on the order-wire circuit.

Fig. 6 is a four-wire order circuit similar to that shown in Fig. 4, and achieves substantially the transmission losses and singing and echo margins identified above regarding both Figs. 1 and 4.. Referring to Fig. 6, the W.-E. and E.-W. transmission paths Ill and I2 including one-way amplifiers H and is are identical with similar circuit elements in Fig. 1; theinput terminals 35 and 39 of the respective W.-E. and E.-W. amplifiers II and i3 are connected through hybrid coil 55 (replacing the bridge network 450i Fig. 5); the output terminals 25 and 3! of the "respective W.-E. and E.-W. amplifiers Ill and [3 are joined through a hybrid coil 56 (replacing the bridge network 48 of Fig. 4) arms 5-! and'6-8 ofthe bridge network 48 are connected to the hybrid coils 55 and 56, respectively; and during non-talking intervals individual resistors may be substituted for the respective telephone subset 47 and 49 by embodying Fig. 5 in Fig. 6 between he lines A--A','and 13-43. It may be demonstrated substantially in the manner specified hereinbefore concerning Figs. 1 and 4 that normal operation of the W.-E. and 'E.-W. amplifiers I! and it meets with tolerable interference from the order-wire circuit of Figs. 5 and 6 during both talking and non-talking intervals on the latter circuit,

Fig, 7 is similar to Fig. 6 except a hybrid coil 5? is substituted for the bridge network 48 of the latter figure by embodying Fig. '7 in Fig. 6 between the corresponding lines AA and B-B. Fig. 3 is similar to-Fig. 6 except a transmitter 58 is applied to the hybrid coil 55 and a suitable signal translator 58 is connected to the hybrid coil 56, both of which connections may be effected by substituting Fig. 8 in Fig. 6 for the circuit portion included betweenthe corresponding lines AA and 3-3 of the latter figure. The hybrid coils 55 and 56 may be so poled that electrical energy from one echo path tends to cancel'the electrical energy from the other path.

Fig. 9 shows an order-wire circuit applied to two-way two-wireeast and west terminals which are coupled to a four-wire circuit of the type shown in Fig. 1 by means of the usual .hybrid coils 6i) and BI. Signal transmission'between the eastand west terminals takes place in the wellknownmanner. Bridge network has arm l-2 connecte'daover path .63 tolthe input terminal 35 of the W.-E. amplifier; arm 3-4 over'path 64' to the two conductors constituting the two-wire two-way output of theeast terminal hybrid coil 6 I; and a resistor'65 across the vertical diagonal [-3. The horizontal diagonal 2-4 of the bridge network 62 is applied to the vertical diagonal l-3 of bridge network 66 embodying in individual arms a telephone device, balancing network and resistors. The horizontal diagonal 2-4 of the bridge network 65 is applied to the vertical diagonal l-3 of bridge network 61 whose hori zontal diagonal 2-4 embodies a resistor, whose arm 1-2 is connected over path 68 to repeat coil 69Jwhich is effectively connected in shunt of the two-wire two-Way output of the west terminal hybridcoil 60, and whose arm 3-4 is connected over path it to repeat coil H which is efiectively connected in shunt oi the two conductors constituting the two-wire two-way output of the east terminal hybrid coil 6|.

In the operation of Fig. 9, the windings of the repeat coils 69 and H are so poled as to be efiectively connected to the output terminals of the one-way W.-E. and E.-W. amplifiers H and I3. Thus, voice current originating in the telephone subset embodied in the bridge network 66 is efiectively applied through the bridge network 62 andover the paths 63 and G4 to the inputs of the one-way W.-E. and E.-W. ampli fiers Hand [3 and the output of the W.-E. am-

plifier I l for transmission over the two-wire lines while incoming voice current on the two-wire lines at the east and west terminals is effectively applied through respective repeat coils 69 and H, paths 68 and I0, and bridge network 61 to the telephone subset embodied in the bridge network 66. Transmission losses and echo and singing margins of the order-wire circuit of Fig. 9. are substantially of the order mentioned above in connection with Fig. 1, and may be individually identified similarly to the manner hereinbefore specified with respect to Fig. 1. Hence, the normal operation of the W.-E. and E.-W; amplifiers II and I3 in Fig. 9 may be accomplished substantially with tolerable interference from the order-wire circuit both during talkingand nontalking intervals on the latter circuit. In the non-talking intervals, it is to be understood that individual resistors maybe substituted for the telephone device and balancing network as pointed out above with respect to Figs. 1 and 2.

Fig. 10 illustrates an order-wire circuit embodying a plurality of multiwinding repeat coils and applied to a four-wire circuit such that the gain of the W.-E. and E.-W. amplifiers II and Ms effective with respect to both transmitting and receiving on the telephone device 82. Referring to Fig. 10, repeat coil '15 has winding [-2 applied to input terminal A of W.-E. amplifier H, repeat coil 16 has winding l-2 connected to output terminal B of the W.-E. amplifier ll, repeat coil 11 has winding I-2 extended to the input terminal C of the E.-W. amplifier l3, and repeat coil 18 has winding I-2 applied to the output terminal D of the E.-W. amplifier l3. Windings 3-4, -6, and 1-8 of individual repeat coils 15, 16.11 and 18 are serially connected to respective networks 19, 80 and BI. Windings 9-H] of the individual repeat coils 15, I6, 11 and 18 are serially extended to the telephone apparatus 82.

The operation of Fig. will be readily understood by referring to Fig. 12 in which the arrows indicate the direction of current flow in corresponding windingsof individual repeat coils I5,-

16, TI, and" when current is initiallycaused to flow from either terminal A, B, C, or Dto the winding l-2 of the repeat coil connected to the particular terminal. Assuming signal current to flow initially from terminal A to the winding |2 of repeat coil 75, other currents are thereby caused to be induced in the remaining windings of the repeat coil 15. Current flows in the same direction in the windings 3-4 of all repeat coils, and therefore flows in the network 19 connected thereto., Current also flows in windings 5-6, 1-8 and 9-H! of allrepeatcoils, and in the networks 80 and 8| and telephone apparatus 82 associated therewith, the amounts of current in" each network and in the telephone apparatus being substantially the same.

Assuming signal current to flow from the terminal B to the winding I-2 of repeat coil 'IBIin Fig. 10, other currents are thereby caused to be induced'in the remaining windings of the repeat coil 76. Current tends to flow in the same-direction in windings 3-4 of all repeat coils, and therefore flows in the network 79 connected thereto. Currentalso flows in windings 5-6,]15-8, and 9-H! of all repeat coils, and in the networks 89 and-8| and the telephone apparatus 82 associated therewith, the amount of current in each network and the telephone apparatus being substantially the same.

Assuming signal current to flow from terminal C to the winding I-2 of repeat coil H in Fig. 10,

other currents are thereby causedto be induced in the remaining windings of the repeat coil 11. a

therewith, the amount of current in each network and the telephone apparatus being substantially the same.

Assuming signal current to flow from terminal D to the winding 1-2 of repeat coil 18 in Fig. '10, other currents are thereby caused to be induced in the remaining windings of the repeat coil 18. Current flows in the same direction in windings 3-4 of all repeat coils, and therefore flows in the network 19 applied thereto. Current also flows in windings 5-6, 1-8 and 9-l0 of all repeat coils, and in the networks 80 and .BI and the telephone apparatus 82, the amount of current flowing in each network and the telephone apparatus being substantially the same.

-Referring to Fig. 12, it will be observed that when signal current is introduced from the terminal A into the winding I-2 of the repeat coil 15, therelative poling of the windings 3-4, 5-6,

1-8, and 9-H! of individual repeat coils-15, Tl

andIB-is such, that current tends to flow in one direction in two Windings but in an opposite direction in the other two windings. As the net magnetic effect of such current flow in the repeat from the terminal B into the winding l-2 of repeat coil 16, thenet magnetic effectin the re peat coils 15, TI and I8 is such that no current is supplied thereby to the respective terminals A, C and D; or when a current is introduced from the terminal C into the winding l-2 of repeat coill-l, the net magnetic eiiect in the repeat cells 15,16 and 18 issuch that 'nocurrent .is lsupplied asoaseo these'repeat coils to. therespective terminals A, B- and D; or when a current is introduced from the, terminal. D. into, thewinding I2 of the repeat coil I8; the net:magneticeffect in the repeat coils I5, 15 and. I1 is such that no current is applied by these repeat coils to the respective terminals A, Band C. This means that the W.-E. am-

plifier, I I will not sing over the loop including repeat 0011515 and IE; that E.W. amplifier I3 will not sing over the loop including repeatcoils I1 and I8; and that echoes will not bejefiectively transmitted over loops embodying any combinationof the repeat coils.I5, I5, I1, and I8- Fig. 11 isa modification of Fig. 10 to the extent that certain multiwinding repeat coils are effectively applied tothe two-wayatwo-wire west and east terminals. Thus, repeat coil I is connected toterminal A which is theinputterminal of W.-E. amplifier II; repeat coil I6 is effectively applied toterminal B which is two-Wire two-way east terminal, or output terminal of the W.-E. amplifier I I; repeat coil I1 is applied to the terminal C which is the input terminalof E.-W. amplifier I3 and. repeat coil. I8. is effectively connected to terminal D which is the two-wire two-way west terminal, or output terminal of the E.-W. amplifier i3. Similarly, to Fig. 10, it can, be shown in Fig. 11 that when current is introduced from individual. amplifier terminals into windings I2 of individual repeat coils, current will fiow in the other individual windings and the networks and telephone apparatus associated therewith; that the W.-E. and 13.-W. amplifiers I I and I3 will not sing through loops including certain repeat coils; and that no echo loops are effective through any combination of repeat coils.

Assuming speech current originates in the telephone apparatus 82 of Fig. 10 and flows in the windings 9I0 of the repeat coils I5, I6, TI, and I8 in a clockwise direction as indicated by the arrows in Fig. 13. This current sets up magnetizing flux in the individual repeat coils. Referring to Fig. 13, it is seen that the poling of the windings of the individual repeat coils is such that current tends to flow in the winding 3-4, 5-6, 'I8, and 9-40 of the respective repeat coils I5, I5, I1, and I8 in certain relative directions. Thus, it is seen that current tends to flow in three windings in the same direction but in a direction opposite thereto in a fourth winding. Hence, a voltage is produced across the individual windings of each repeat coil, and is supplied from the windings I2 of each repeat coil to the respective terminals A, B, ,C, and D to which the former are connected. Further, it is seen in Fig. 13 that the current in windings 34 of repeat coils I5 and 11 tends to flow in the same direction but in a direction opposite thereto in repeat coils I6 and I8. directed currents in the windings 3-4 cancel each other and therefore no speech power is absorbed in the network I9, Fig. 10, connected to these windings. Likewise, current in windings 5-6 of repeat coils 15 and I8 tends to fiow in the same direction but in a direction opposite thereto in repeat coils I6 and 11. Thus, the voltages occasioned by such oppositely directed current in the windings 5-6 cancel each other, and therefore no speech power is absorbed in the network 80, Fig. 10, applied to these windings. Similarly, current in the windings IB of repeat coils I5 and I5 tends to flow in the same direction but in a direction opposite thereto in repeat coils TI and I8. Consequently, the voltages caused by such oppositely directed current in the windings Hence, the voltages due to such oppositely I-.8 cancel each other, and therefore no speech power is absorbed in the network 8|, Fig. l0, connected to the windings 'I8. Similarly, it can be demonstrated with Figs. 11 and 13 that current originating in the telephone apparatusBZ is effectively transmitted through repeat coils I5, I6, 11 and I8 to the effective terminals A, B, C, and D associated with the respective W.-E. and E.-W. amplifiers II and I3; and further that no speech power is absorbed in the networks 19, 80, and BI i It is to be understood that the embodiments of the invention illustrated in Figs. 4,6, 9, l0 and 11 may also include resistance pads inthe individual paths interconnectin the one-way paths or twoway two-wire circuits similarly to the resistance pads 24, 34, 3S and 38 shown in Fig. 1; and Iurther that the bridging loss of the order-wirecircuits is compensated for inthe gain setting of the amplifiers, and is preferably keptv constant by. replacing thetelephone devices with equivalent loss devices when the order-wire circuits are not in use at individual stations, as above pointed out.

What is claimed is: 1. In combination in a four-wire signal transmission circuit comprising two oppositely directed one-way signaltransmitting paths, each embodying at least one one-way amplifier having input and output terminals, and means to interconnect said pathsfor telephoning and non-telephoning conditions such that interference with the normal operation of said amplifiers in both said oneway paths for signal transmission is limited substantially to a tolerable amount, comprising a bridge network adaptable for telephoning and non-telephoning conditions, a hybrid coil applied to each diagonal of said bridge network, and circuit means to connect each of said hybrid coils to both said one-way paths at opposite corresponding terminals of said amplifiers embodied therein.

2. The combination accordingto claim 1 in which said bridge network embodies a resistor in each arm, and is thereby adapted for the non-telephoning condition.

3. The combination according to claim 1 in which said bridge network embodies a telephone device in one arm, a balancing network in an adjacent arm, and individual resistors in the remaining two arms, and is thereby adapted for the telephoning condition such that the speech energy originating in said telephone device is applied initially to said input terminals of said amplifiers in both said one-way paths.

4. The combination according to claim 1 in which said circuit means connects one hybrid coil to said output terminals of each amplifiers embodied in both said one-way paths and the other hybrid coil to said input terminals of said amplifiers embodied in both said one-way paths.

5. The combination according to claim 1 in which said circuit means connects one hybrid coil to said output terminals of said amplifiers embodied in both said one-way paths and the other hybrid coil to said input terminals of said amplifiers embodied in both said one-way paths, said portion of the circuit means connecting said one hybrid coil to said output terminal of said amplifier in one of said two one-way paths being reversed with respect to the further portion of said circuit means connecting said one hybrid coil to said output terminal of said amplifier in the other of said two one-way paths.

6. In combination in a four-wire signal system,

' two oppositely directed one-way paths for transmitting signal current, each path embodying at least one one-way amplifier having input and output terminals, and means to interconnect said two paths for telephoning and non-telephoning conditions such that" interference with the normal operation of said amplifiers in said two paths-for signal transmission is limited substantially to a tolerable amount, comprising a bridge network adaptable for telephoning and non-telephoning conditions, a hybrid coil applied to each diagonal of said bridge network, circuit means to connect one hybrid coil to said output terminals of said amplifiers in said two paths, and further circuit means torconnect the other hybrid coil to said input terminals of said amplifiers in said two'paths.

.7, In combination in a four-wire signal system, two oppositely directed one-way signal transmission paths, each embodying at least one one-way amplifier having input and output terminals, and means for applying voice current to said input terminals of said amplifiers in said two paths and receiving voice current from said output terminals of said amplifiers in said two paths and at the same time for limiting interference with thenormal operation of said amplifiers insaid two paths for signal transmission substantially to a tolerable amount, said means 7 comprising a bridge network embodying a telephone device in one arm, a balancing network in another arm, and individual resistors in-each oithe remaining two arms, a pair of multiwinding hybrid coils, each of which is applied to one diagonal of said bridge network, circuit means to connect one hybrid coil to said outputterminals of said amplifiers in said two paths for receiving incoming speech energy therefrom, and

further circuit means to connect the other hybrid coil to said input terminals of said amplitwo of said terminals to said apparatus, said circuit means also connecting said two terminals and the remainder of said terminals to said input and output circuits. of bothrsaid lines whereby the voice currents originating in said apparatus are applied to said input circuits inboth said lines for outgoing transmission thereon, and the voice currents incoming on both said lines are picked up from said output circuits of both said linesand applied to said apparatus.

PAUL G. EDWARDS. 

